Image sensing apparatuses are roughly classified into a CCD sensor and CMOS sensor. The CCD sensor sequentially transfers photoelectrically converted charges in the vertical and horizontal directions. In the CMOS sensor, each pixel has an amplifier for amplifying a photoelectrically converted signal, and the amplified signals are sequentially output using vertical and horizontal scan circuits.
An image sensing apparatus is often required of an operation for reading out pixels at a high frame rate (speed) although the resolution lowers, in addition to a standard operation for reading out all pixels.
The CMOS sensor can attain random access scans by controlling the scans of the vertical and horizontal scan circuits, and can also scan while decimating pixels. In such case, since information of skipped pixels is discarded, it is disadvantageous for sensitivity.
Hence, a method that couples the outputs of neighboring photoelectric conversion units such as photodiodes via switches to add charges, in other words, to calculate the average value of potentials, and outputs the average has been proposed.
FIG. 19A is a circuit diagram for four pixels extracted from an image sensing apparatus. In FIG. 19A, a reset switch for each photodiode PD is not shown.
As shown in FIG. 19A, the cathodes (outputs) of four photodiodes PD are coupled by turning on add switches SW30, SW31, and SW32 to add charges, a pixel is selected by turning on a select switch SW20, and the average value of potentials is output from an amplifier Tr20 onto a vertical output line VL.
As another method, as shown in FIG. 19B, after a charge on a photodiode PD is transferred to each capacitor FD by turning on a transfer switch SW21, the transfer switch SW21 is turned off, and the capacitors FD are coupled by turning on add switches to add charges. Then, a pixel is selected by turning on a select switch SW20, and the average value of potentials is output from an amplifier Amp20 onto a vertical output line VL.
In general, since each photodiode PD accumulates a small amount of charge upon receiving light, the capacitance of the photodiode PD or the capacitor FD shown in FIG. 19B is preferably set to be as small as possible so as to obtain a high potential.
On the other hand, each add switch SW or its connection wire has a stray capacitance CSW in no small measure, which lowers sensitivity and also increases kTC noise.
Also, each add switch SW or its connection wire suffers a leakage current ISW. With this current, an important photocharge leaks. Also, noise increases due to shot noise generated in association with such leakage current.
That is, the add switch SW or its connection wire, which is added for the purpose of increasing sensitivity, lowers sensitivity.
FIG. 20 shows another example of the conventional addition method among pixels in an image sensing region of an image sensing apparatus. In the image sensing apparatus shown in FIG. 20, add switches are inserted between neighboring pixels P, and decoders D for controlling the corresponding add switches are provided in correspondence with the add switches. Furthermore, a plurality of control lines CL run in the image sensing apparatus to control the decoders D. In an addition mode of the image sensing apparatus, the decoders D are controlled to individually turn on/off the add switches.
However, when the add switches are inserted between neighboring pixels and switch controllers such as decoders and control lines are provided for the respective add switches, an addition circuit occupies a large area in the image sensing region.